Many prior attempts have been made to improve the salt spray protection on metal parts and include various protective coatings, or combinations of more than a single coating, on a metal part. U.S. Pat. No. 4,165,242 discloses a general description of prior art processes for treating phosphated metal parts to improve corrosion resistance utilizing the general steps of applying a paint coat, and then an oil coat in separate steps. This patent includes descriptions of some of the paint and oil formulations well known to those skilled in the art to be useful in such processes.
Initial attempts to use an oil coupled into the paint vehicle resulted in limited corrosion resistance which was not acceptable to industry standards. Under this prior art process, the metal parts were treated in a batch method. A first coat of paint was applied to the parts by dip spinning them. The parts were then baked, after which any parts adhered to one another had to be physically separated. However, this separation often resulted in the paint coat being pulled away from one of the parts at the point of engagement of adjacent parts resulting in exposing an unpainted surface area.
To combat this problem a second painting step was employed. Of course, there still existed the possibility that a limited amount of surface area would still remain exposed. After the second painting step, the entire batch was coated with an oil emulsion to reduce the affect of corrosion. This process involved high paint VOC values, substantial labor, and a significant investment in time, all of which resulted in parts which still had a tendency to corrode due to uncoated surface area.
In an attempt to solve that problem, a waterborne system was invented having a lower VOC value, and which system was more effective. U.S. Pat. No. 4,737,385 discloses a composition and method whereby a single application to a part of a coating comprising an oil phase and a resin film forming paint advantageously results in providing both a paint film layer and a distinct oil layer overlying the paint film. Significantly improved corrosion resistance is disclosed therein, along with the economical advantage of eliminating the plural separate coating steps heretofore conventionally employed.
However, for some machine parts, such as threaded torque fasteners, it is desirable to provide a reduction of the coefficient of friction, such that the desired tension of the fastener can be achieved with reduced torque requirements. For years, cadmium electroplate and wax coatings were considered the standard for achieving low levels of friction for lubricated torque fasteners. However, cadmium became classified as a hazardous waste material due to its high toxicity. Therefore, it became desirable to replace the cadmium electroplate and wax coating on parts, yet still retain corrosion resistance and low coefficients of friction.
U.S. Pat. No. 4,849,264 discloses such a coating composition. However, while excellent corrosion resistance is provided to metal parts along with the parts being able to torque, the overlying oil coat may be considered objectionable in some applications due to its "oily feel" or to the residue of the oil coat which is left on the hands of one touching the coated part. In some individuals, the presence of the residue can be accompanied by skin irritation. Thus, it was desirable to formulate a protective coating similar to that disclosed in U.S. Pat. No. 4,737,385 which could be substantially volatilized during the curing step to leave only a dry-to-touch film.
U.S. Pat. No. 5,037,678 discloses such a dry-to-touch coating composition. In order to obtain a dry-to-touch product, it was necessary to volatilize the oil associated with the product. The oil associated with U.S. Pat. Nos. 4,737,385 and 4,849,264 volatilized at 300.degree. F. However, at this temperature the paint component of the coating was destroyed by fracturing. This destruction of the paint component compromised the corrosion resistance associated with the dry-to-touch product. Therefore, an oil having a lower volatilization temperature was deemed necessary. However, when the parts were baked, the high percentage of oil loss also resulted in the parts having a tendency to stick together. Therefore, the invention disclosed in U.S. Pat. No. 5,037,678 incorporated an amine soap which formed a parting agent on the coated surface so as to prevent the sticking of the parts.
However, the coating composition disclosed in U.S. Pat. No. 5,037,678 did not have torque properties. It is this apparent that the need exists for an improved coating composition which provides effective corrosion resistance while at the same time exhibiting torque.